Sophisticated wireless data communications devices, systems, and networks are bringing about the increasing need for higher data rates and the support of greater numbers of users and data traffic. As a consequence these networks employ techniques such as MIMO signal coding for achieving higher bandwidths, wherein multiple parallel radio frequency (RF) receiver and transmitter chains are used together to transmit a single data stream. Such techniques, however, increase the complexity of testing wireless devices. Manufacturers, vendors and users therefore have a greater need for better testing of such MIMO transmission systems.
Current state of the art in wireless technologies, such as the IEEE 802.11 WLAN protocol, support not only MIMO transmission and reception but also include advanced signal processing techniques such as beamforming. Beamforming improves the reception of transmitted MIMO signals at a particular MIMO receiver by pre-conditioning the transmitted signals to increase the signal-to-noise ratio (SNR) at that specific receiver's spatial location, while concurrently decreasing the signal-to-noise ratio at other locations where the signal is not desired to be received. Beamforming is accomplished by pre-processing transmitted signals using a precoding matrix selected according to the characteristics of the RF channel existing between the MIMO transmitter and the target MIMO receiver. The precoding matrix is structured to pre-emphasize specific paths between a set of transmit antennas at one spatial location and a set of receive antennas at a different spatial location. At the receiver, the MIMO decoding process will remove the effect of the precoding matrix, and thereby enhance the SNR for the given transmitted signal between those two spatial locations, while diminishing the SNR from other locations.
With reference to FIG. 1, a representational view of a MIMO transmitter 10 and MIMO receiver 11 is shown in an “over the air” RF environment containing metallic scatterers 12. Multiple RF paths 13 are formed between the antennas of the transmitter and receiver due to the scatterers, enabling MIMO communications to take place. When MIMO systems are used in an “over the air” environment such as depicted in FIG. 1, the signals sent by the set of transmit antennas (after precoding) form a composite signal that is received by the set of receive antennas. The signal processing operations applied during the decoding process are structured such that they may be applied to the received composite signals in any order; there is no one-to-one correlation between a particular transmit antenna and a particular receive antenna.
However, when a MIMO system is placed in a test environment, where cables are typically used to interconnect receive and transmit antennas, the situation becomes different. FIG. 2 represents such a test environment, where MIMO transmitter 20 communicates with MIMO test receiver 21 using RF cables 22. As shown in the figure, the antennas are removed from the antenna ports of transmitter 20 and test receiver 21 (the ports being shown in the figure as T1, T2, T3, T4 and R1, R2, R3, R4 respectively). In this case there is a one-to-one correspondence between each RF transmitter antenna port and each RF receiver antenna port. In this case, the signal received by each receive antenna port is not a composite of all the signals, and hence there is a one-to-one correlation between a particular transmit antenna and a particular receive antenna that must be taken into account.
Turning now to FIG. 3, a partial block diagram of an Orthogonal Frequency Division Multiplexing (OFDM) MIMO transmitter 20 and a MIMO test receiver 21 interconnected by RF cables 22 is shown. MIMO transmitter 20 contains encoder 30 that encodes digital data with some type of error-correcting code such as a binary convolutional code (BCC), followed by stream parser 31 that splits the encoded data stream into multiple independent binary streams. Constellation mappers 32, 33, 34, 35 then perform digital modulation on the binary streams to create OFDM symbols. Cyclic shift delay (CSD) functions 36, 37, 38 delay some of these symbol streams by variable amounts, before passing them to transmit precoder 39 for performing beamforming precoding according to the measured characteristics of the RF channel. The precoded digital symbols are passed to spatial mapper 40, which implements the MIMO space/time mapping procedure, after which the data is processed by inverse Fast Fourier Transform (IFFT) units 41, 42, 43, 44 which transforms the data from the frequency domain to the time domain. Finally, the time domain signals are converted to analog RF outputs by RF transmit processors 45, 46, 47, 48 and transmitted on cables 22.
MIMO test receiver 21 receives the RF signals on cables 22 via RF receive processors 50, 51, 52, 53, which down-convert and digitize them to output baseband signals, which are in turn passed to FFT processors 54, 55, 56, 57 to transform them from the time domain to OFDM symbols on frequency domain subcarriers. The frequency domain signals are equalized by MIMO equalizer 58 to remove the effects of the RF channel, as measured by channel estimator 59, and then passed to constellation demappers 63, 64, 65, 66 to recover the original binary data streams. Some of these symbol streams are delayed by variable amounts using inverse CSD (CSD−1) functions 60, 61, 62, compensating for the CSD 36, 37, 38 applied in MIMO transmitter 20. Finally, the individual binary data streams are combined using stream deparser 67 and then processed by decoder and FEC unit 68, which reverses the encoding applied at the transmitter and also corrects any bit errors that may be encountered, producing a copy of the original digital data supplied to MIMO transmitter 20.
The use of RF cables 22 to interconnect MIMO transmitter 20 to MIMO test receiver 21 is advantageous from the point of view of excluding unwanted interference and noise, reducing variable path losses encountered in “over the air” environments, and in general improve repeatability and controllability during testing. However, precoder 39 and CSD functions 36, 37, 38 impose certain specific characteristics on each transmitted symbol stream that are different from the other symbol streams; for example, signals transmitted on antenna port T1 in FIG. 3 must be received at antenna port R1, and so on. This is necessary so that receiver 21 can successfully decode and extract data from the various symbol streams.
As has been mentioned, in an “over the air” environment there are no RF cables interconnecting the MIMO transmitter and receiver, and all MIMO receiver ports receive a composite of the signals transmitted by each transmitter port. MIMO test receiver 21 is thus able to treat all of its antenna ports R1, R2, R3, R4 identically and rely on the channel estimation process to determine which antenna port belongs to which RF chain. However, in a cabled environment this is not the case.
Turning now to FIG. 4, an example of an inadvertent mis-cabling of RF cables 80 between MIMO transmitter 20 and MIMO test receiver 21 is shown. In this case, T1 on MIMO transmitter is connected to R3 on MIMO test receiver 21, and so on. This renders it difficult or impossible for MIMO test receiver 21 to successfully receive and decode the MIMO data, as the wrong CSD values and the wrong beamforming precoding matrix will be applied to the incoming signals under the assumption that the receive signals have a particular order corresponding to the expected transmit signals that would normally be coupled to the receiver ports.
Unfortunately it is not always simple to ensure that the interconnection between MIMO transmitters and test receivers follows the expected order. For example, the manufacturer of MIMO transmitter 20 may not mark the significance of the various antenna ports, under the expectation that during normal use these ports are connected to antennas for “over the air” operation. It may also be possible that testing may have to be performed on MIMO transmitters 20 with internally configured antennas (i.e., without easily accessible external antenna ports), in which case no labeling may exist. It may even be possible that the meaning of signal labels assigned by a particular manufacturer may not correspond to the meaning of the same signal labels assigned by some other manufacturer, as there is no known industry standard for labeling MIMO transmitter antenna ports. In all cases, the net result is that the test may fail because MIMO transmitter 20 is incorrectly connected to MIMO test receiver 21, as depicted for an exemplary case in FIG. 4.
It should be apparent that the issue of proper ordering also occurs in the reverse direction, i.e., from a MIMO test transmitter to a MIMO receiver under test. An aspect that should be noted, however, is that within a particular device the ordering of transmitted signals and received signals relative to the set of antenna connectors is the same; that is, if an antenna connector carries a particular segment of transmitted signal, such as for example that generated by IFFT 41 and RF transmit processor 45 in FIG. 3, then that antenna connector will also accept a corresponding segment of received signal, such as for example that accepted by RF receive processor 50 and FFT 54 in FIG. 3. Therefore, determining the order of transmitted signals will assist in determining the order of received signals.
The known prior art in the field of wireless device testing therefore suffers from serious shortcomings with regard to coupling a MIMO DUT to a MIMO wireless tester. There is hence a need for improved wireless MIMO test systems and methods. A test system that can automatically determine the assignment of transmit antenna ports to transmitted signals is desirable. It is preferable for such a system to automatically adapt to the ordering of transmitted signals, so as not to require an operator to recable or reconfigure the test setup. Finally, such a system should automatically generate transmitted signals to a DUT with the same ordering as the signals received from the DUT, thereby ensuring that two-way communications can be performed without recabling or reconfiguring.